Due to the recent progress in the art of photographic materials for photograph-taking use, newly developed photographic materials of high photographic speed have been highly commercialized. The expansion of the photographing environment depends on the attainment of high photographic speed in photographic materials, for instance, photographing in a dark room without a strobe light, photographing of., e.g., sports scenes, through a telephoto lens while rapidly handling the shutter, photographing requiring many hours of exposure, e.g., taking astrophotographs, and so on.
For the purpose of increasing the photographic speed of a photosensitive material, considerable efforts have been expended. A great number of methods for forming silver halide grains having a desired form and composition, chemical sensitization, spectral sensitization, additives, coupler structures, and so on have been developed. One method involves combining a method of enlarging the size of the silver halide grains with another method of increasing the photographic speed. This method has been a typical measure for producing a photosensitive material of high photographic speed in the photographic arts. However, the progress of the art of photography is still behind the requirements for photosensitive materials of a high photographic speed.
More specifically, although enlargement of the size of the silver halide emulsion grains can increase the photographic speed to some extent, it necessarily leads to a decrease in the number of silver halide emulsion grains, provided that the content of silver halide in the emulsion is maintained constant. As a results, the number of development initiation centers is decreased. Therefore, the increase in size of the silver halide grains entails a disadvantage in that the graininess is greatly spoiled. In order to offset this disadvantage, various methods have been proposed. For instance, a method of using a photosensitive material containing at least two emulsion layers which has the same color sensitivity, but different photographic speeds, that is, different grain sizes, respectively, as described in British Patent No. 923,045 and Japanese Patent Publication No. 15495/74; a method of using a rapidly reacting coupler, as described in Japanese Patent Application (OPI) No. 62454/80 (the term "OPI" as used herein means an "unexamined published application"); a method of using a so-called DIR coupler or DIR compound, as described in U.S. Pat. Nos. 3,227,554 and 3,632,435; a method of using a coupler capable of producing a diffusible dye, as described in British Patent No. 2,083,640; a method of using silver halide grains having a high mean silver iodids content, as described in Japanese Patent Application (OPI) No. 128443/85; and so on are well-known. Although these methods each has a great effect and can be said to be an excellent invention, they are still insufficient to meet many of the requirements for heightening both the photographic speed and the image quality. Therefore, in order to increase the grain size of the silver halide emulsion and at the same time, to increase the number of development initiation centers to as large as possible, high-speed color negative photosensitive materials have been designed to contain silver halide emulsion grains in the largest amount as possible so that various properties, such as the desilvering capacity at the time of bleach-fix processing, are not adversely effected.
It has recently been noted that high sensitivity photographic materials are unsatisfactory for the following reasons: First, these photographic materials often suffer from a deterioration of the photographic characteristics, including an increase of fog, a lowering of sensitivity, a deterioration of graininess, etc., over the course of time after the manufacture thereof until the use thereof. Second, the sharpness can not be sufficiently elevated, and hence the photographic materials can not meet the severe requirements as to the image quality of recent photographic materials. Third, the pressure-resistance is poor, and hence there is a high danger of causing unfavorable sensitization of the photographic materials under the present-day situation where the processing step in the laboratory is being speeded up and where automatic film-winding devices in cameras are being employed. Fourth, there also is a high danger of an insufficient fixation or insufficient silver-removal arising in the conventional high sensitivity photographic materials, since the developing time is being shortened and the amount of the processing solution replenisher is also being reduced in today with the popularization of mini-laboratories.